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1. Field of Invention
This invention relates to a process for selective hydrogenation of an olefinic feed stream containing acetylenic and diolefinic impurities. More particularly, this invention relates to a process for selective hydrogenation of a C2 and C3 olefinic feed stream containing acetylenic and diolefinic impurities without first separating the C2 olefins from the C3 olefins, whereby the hydrogenation of the acetylenic and diolefinic impurities occurs in a single processing step. This invention also relates to a catalyst and to its use in this process for selective hydrogenation.
2. Prior Art
The manufacture of unsaturated hydrocarbons usually involves cracking various types of hydrocarbons and often produces a crude product containing hydrocarbon impurities that are more unsaturated than the desired product. These unsaturated hydrocarbon impurities are often very difficult to separate by fractionation from the desired product. The most common example is ethylene manufacture, in which acetylenes are common by-products. In a similar way, formation of propylene produces C3 H4 (methyl acetylene or allene) and butadiene. Further, it has often been difficult industrially to remove such undesirable, highly unsaturated hydrocarbons by hydrogenation so that no significant hydrogenation of desired hydrocarbon takes place. One example of this catalytic process is described in UK Pat. No. 916,056.
Two general types of gas phase selective hydrogenation processes for removing undesired, unsaturated hydrocarbons have come into use. One, known as xe2x80x9cfront-endxe2x80x9d hydrogenation, involves passing the crude gas from the initial cracking step, after removal of steam and condensible organic material, over a hydrogenation catalyst. Despite the large hydrogen content of such gas, which is very greatly in excess of the acetylenes and sufficient to hydrogenate a substantial part of the olefin present, operation with sufficient selectivity to produce olefins of polymerization quality is well established and catalyst lives of many years are obtained. In the other type of gas phase selective hydrogenation, known as xe2x80x9ctail-endxe2x80x9d hydrogenation, the crude gas is fractionated and the resulting concentrated product streams are individually reacted with hydrogen in a slight excess over the quantity required for hydrogenation of the highly unsaturated hydrocarbons which are present. However, in tail-end use there is a greater tendency for deactivation of the catalyst, and consequently, periodic regeneration of the catalyst is necessary.
It has been proposed to employ both front-end and tail-end selective hydrogenation in the same plant.
Further, although the above-mentioned processes are normally carried out in the gaseous phase, it has also been proposed to perform these hydrogenation reactions with the hydrocarbon in a liquid phase, especially when the feed contains compounds containing four or more carbon atoms in the molecule.
A number of patents have discussed selective hydrogenation of such unsaturated hydrocarbons, including U.S. Pat. Nos. 4,126,645, 4,367,353, 4,329,530, 4,347,392 and 5,414,170.
Catalysts that have been found to be suitable for such selective hydrogenation reactions include palladium supported on an alumina substrate, as disclosed for example in U.S. Pat. Nos. 3,113,980, 4,126,645 and 4,329,530. The catalyst used for these reactions is normally formed as shaped pieces, such as pellets. The palladium on alumina catalysts generally have a surface area in the range of 5 to 200 m2/g and a pore volume of at least 0.4 cm3/g, with the radii of the pores greater than 300 angstroms. The amount of the palladium present in the catalyst is generally in the range of about 0.1 to about 1 percent, as disclosed in U.S. Pat. No. 4,367,353 and from 0.01 to 5 percent, as disclosed in U.S. Pat. No. 3,113,980.
Other types of gas phase palladium on alumina catalysts for the selective hydrogenation of acetylene compounds are disclosed, for example, in U.S. Pat. Nos. 5,925,799, 5,889,138, 5,648,576 and 4,126,645.
Current tail-end, selective hydrogenation technology for C2 and C3 feeds employs two separate and distinct processing lines, one for the selective hydrogenation of C2 olefin compounds and one for the selective hydrogenation of the C3 olefin compounds. The C2 compounds with their related contaminants are first separated from the C3 compounds and their related contaminants. This separation results in the formation of two distinct feed streams. Each of these feed streams is then independently reacted with hydrogen and carbon monoxide in the presence of a palladium catalyst to selectively hydrogenate the respective C2 and C3 contaminants.
The current tail-end technology has several deficiencies including (1) higher production of heavy oligomers resulting in shorter operating cycles, (2) lower average selectivity, (3) the need for frequent regeneration, and (4) the need for the introduction of carbon monoxide to the process feed stream to improve catalyst selectivity. (The carbon monoxide must be subsequently removed from the feed stream in a further processing step.) In addition, (5) this two-stage process is more expensive to perform than would be a single-stage process, at least partially because of increased capital costs, and can result in the formation of certain longer chain by-products resulting in loss of the desired olefin end products.
Accordingly, it is an object of this invention to disclose a process for the selective hydrogenation of a C2 and C3 olefinic feed stream containing C2 and C3 acetylenic and diolefinic impurities.
It is a further object of the invention to disclose a process for the selective hydrogenation of a C2 and C3 olefinic feed streams containing C2 and C3 acetylenic and diolefinic impurities without first separating the C2 olefins from the C3 olefins, thereby utilizing a single process step, whereby C2 and C3 acetylenic and diolefinic impurities are removed in a single process step.
It is a still further object of this invention to disclose a catalyst useful for the single-stage, selective hydrogenation of a C2 and C3 olefinic feed stream containing C2 and C3 acetylenic and diolefinic impurities in a single process step.
It is a still further object of this invention to disclose process steps for a single-stage selective hydrogenation of a C2 and C3 olefinic feed stream containing C2 and C3 acetylenic and diolefinic impurities, whereby the quantity of the desirable C2 and C3 olefins is not substantially reduced.
It is a still further object of this invention to disclose the process conditions for a single-stage hydrogenation of a C2 and C3 olefinic feed stream containing C2 and C3 acetylenic and diolefinic impurities.
These and other objects can be obtained by the disclosed process for the selective hydrogenation of a C2 and C3 olefinic feed stream containing C2 and C3 acetylenic and diolefinic impurities which is disclosed by the present invention.
The present invention is a process for the single-stage selective hydrogenation of a C2 and C3 olefinic feed stream containing acetylenic and diolefinic impurities comprising
preparing a C2 and C3 olefinic feed stream, which stream may include acetylenes, diolefins and certain low molecular weight, light-end gases selected from the group consisting of hydrogen, carbon monoxide and methane;
separating from the feed stream the light end gases to generate a C2 and C3 olefinic feed stream which includes C2 and C3 acetylene and diolefin impurities; hydrogenating the C2 and C3 acetylene and diolefin impurities in a single feed stream over a catalyst bed by the addition of hydrogen to the feed stream to yield a C2 and C3 olefinic feed stream with substantially reduced quantities of C2 and C3 acetylene and diolefinic impurities while not substantially reducing the quantities of the C2 and C3 olefins in the feed stream; and
separating the C2 olefins from the C3 olefins.
In a preferred embodiment of the process for the single-stage selective hydrogenation of a C2 and C3 olefinic feed stream containing C2 and C3 acetylenic and diolefinic impurities as discussed above, the feed stream comprises from about 15 to about 50 percent by weight ethylene, from about 10 to about 25 percent by weight ethane, from about 25 to about 50 percent by weight propylene and from about 10 to about 25 percent by weight propane.
In a further preferred embodiment, the single-stage selective hydrogenation process as discussed above is utilized in a xe2x80x9ctail-endxe2x80x9d selective hydrogenation unit.
In another preferred embodiment the catalyst useful for the process for the single-stage, selective hydrogenation of a C2 and C3 olefinic feed stream containing C2 and C3 acetylenic and diolefinic impurities comprises a palladium on alumina catalyst.